Chapter II: constants, variables and data types

　2: Constant

　　2.1 Overview

　　Constant: refers to the Java program in fixed data.

　　2.2 classification　　

Types of	meaning	Data example
Integer constant	All integer	0,1,2
Decimal constant	All decimals	0.1,0.2，-0.1
Character constant	Single quotation marks, can only write a character, you must have content	'A', '', 'good'
String constant	Double quotation marks, you can write more than one character, you can not write	"A" ，"Hello" ，"你好" ，""
Boolean constants	Only two values ​​(flow control to explain	true ，  false
Constant air	Only one value (reference data types to explain	null

 

 

 

 

 

 

 

 

　　2.3, output various types of constants

 

 public  class  ConstantDemo { 
     public  static  void  main (String [] args) {               // output integer constant System.out.println (123);          // output decimal constant System.out.println (0.125);          // output character constants System. Out.println ( 'A');          // output Boolean constants System.out.println (to true);          // string literals System.out.println ( "Hello the Java");    
   } 
}

 

　3, variable

　  Overview 3.1, variables

　　Variable: Constant data is fixed, then the amount of change in the program can be called variables.

　　3.2 Notes

　　Variable Name: braces in the same range, the variable name can not be the same.

　　Variable assignment: defined variables, no assignment can not be used.

　　3.3 definitions, variable

　　Data type variable name = data value

   4 , the data type

　　4.1, the data type classification
    　   Java data types are divided into two categories: 
    　　basic data types include: integers, floating point, character, Boolean. 
    　　Reference data types: a class, arrays, interfaces.
　　4.2, the basic data types　

type of data	Keyword	Memory footprint	Ranges
Byte	byte	1 byte	-128-127
Short integer	short	2 bytes	-32768-32767
Integer	int	4 bytes	-231 th ~ 31 th power of 2 -1
Long integer	long	8 bytes	-2 63 63 th power to 2 -1
Single-precision floating-point	float	4 bytes	1.4013E-45~3.4028E+38
Double-precision floating-point	double	8 bytes	4.9E-324 ~ 1.7977E + 308
Character	char	2 bytes	0-65535
Boolean	boolean	1 byte	true，false

　　

 

 

 

 

 

 

 

 

 

 

　　

　　Defines all the basic data types of variables, as follows: